You might want to read Debian interfaces see the bottom section for the details. In summary, just add multiple 'interface' sections with static addresses and skip the gateway definition. Typically defining them as /128 is a good idea btw even though the real interface is a /64. Note that you will run into a lot of fun with outbound addresses, as a random one more or less is chosen... (depends typically on the order of add and/or on the deprecate flag if code is good).